Defects from Raw Materials and Batching Errors

Some folks use materials right out of the ground to make ceramics. (If you don't have clay or shale where you live, there is no brick manufacturer up the road.)
Brick manufacturers usually mine clay by the open pit method. That means that they usually don't tunnel for the clay. They carefully remove the overburden (the dirt, weeds, trees, old cars, and what-have-you on top) leaving a clean clay or shale face. Then they mine the clay or shale and deliver it to the factory.
(See Ceramics: Industrial Processing & Testing, John T. Jones & M. F. Berard, Iowa State University Press)
In other words, brick manufactures have a one component composition (the clay or shale). Clays and shales happen to have some sulfate content. You might get blistering or scumming or some other unpleasant defect.
Some other manufacturers may add barium sulfate such as the sanitary ware manufacturers that manufacture your bathroom fixtures. Vanadium compounds cause scumming in otherwise pretty white bricks.
Defects caused by raw materials are usually related to impurities or particle size. The defect can turn up at any time in the process. Impurity Defects
Lignite
Lignite is common in many clays. It can be hard anthracite or softer bituminous coal or lignite which is softer than the first two.
Screening can reduce lignite from slurries.
Clay companies have some control over the size and amount of lignite in their clays (often by blending.
You can run a screen analysis on the material and see how much lignite remains on the screens. If it is higher than the previous shipments, call the clay company and say these words: "What's with all this lignite!"
Grit
I was new with the company but not inexperienced in solving material problems.
The representative from a kaolin company asked, "What's the grit?"
The number represented the amount of coarse material that showed up during a screen analysis of the clay. For this particular clay, the grit was in the form of mica.
Iron and Manganese Compounds, Silicon Carbide, Soy Beans and Salts
Sometimes clay manufacturers ship clays to storage areas by rail, ship, or barge. Dockside raw material storage is always dangerous for contamination. The reason is that these facilities ship iron ores, ferrosilicon, silicon carbide and they are not very careful about cleaning out a shed of ferrosilicon or other contaminant before loading it with a shipload of clay. (We changed from bulk to Super Sack® shipments.)
That occurs at some characteristic temperature.
We used the same British china clay in two different plants.
In one plant we had blisters in our decorated ware, the worst possible condition.
The other factory using the same clay didn't. Why? The bisk temperature was much higher in the not affected plant. The contaminant, a manganese compound, decomposed before glazing and decoration.
The affected plant had both lower bisk and glost temperatures. The problem was complicated because we had to prove that the supplier caused a heavy loss that their insurance company should pay.
To isolate a contaminant before you send samples out, elutriate clays and screen non clays.
To elutriate a clay, take about ten pounds of clay and keep washing it down until only the contaminant is left. Chloride can be detected from salts by washing the clay with deionized water and testing with a soluble silver nitrate solution. Salts cause blisters!
The clay picked up the beans from a hopper car. Particle Size
One of the most serious formulating errors is in not controlling particle size in the batch recipe.
For example, if you use too much of a certain clay having a very fine particle size, you will have problems. Slip casting requires a coarser particle size distribution than a plastic forming process for the same formulation. You must use some coarser grained clay(s) in your casting process. If you want to have control of your casting process, then use a coarse-grained kaolin plus a fine-grained kaolin, and a coarse-grained ball clay plus a fine-grained ball clay. Changing the ratio of fine to coarse clays will give you control. You must maintain the total amount of kaolin and the total amount of ball clay to preserve color of the product.
Look at the particle size distribution of the clays you are using in your process.